lst= Range[14]
{1,2,3,4,5,6,7,8,9,10,11,12,13,14}
A list of positions in lst ( for your purpose Range[1, Length[lst], 3] will
do)
ps= {3,5,10};
The following applies h to each ps element in lst and adds the result to the
following element
(lst[[ps+1]]=h/@lst[[ps]]+lst[[ps+1]];lst)
{1,2,3,4+h[3],5,6+h[5],7,8,9,10,11+h[10],12,13,14}
--
Allan
---------------------
Allan Hayes
Mathematica Training and Consulting
Leicester UK
www.haystack.demon.co.uk
hay at haystack.demon.co.uk
Voice: +44 (0)116 271 4198
Fax: +44 (0)870 164 0565
"AES" <siegman at stanford.edu> wrote in message
news:anufl8$ap2$1 at smc.vnet.net...
> I want to apply a function to every k-th element of a long list and
> add the result to the k+1 element.
>
> [Actually k = 3 and I just want to multiply myList[[k]] by a
> constant (independent of k) and add the result to myList[[k+1]] for
> every value of k that's divisible by 3.]
>
> Is there a way to do this -- or in general to get at every k-th
> element of a list -- that's faster and more elegant than writing a brute
> force Do[] loop or using Mod[] operators, and that will take
> advantage of native List operators, but still not be too recondite?
>
> I've been thinking about multiplying a copy of myList by a "mask list"
> {0,0,1,0,0,1,..} to generate a "masked copy" and approaches like that.
> Better ways???
>